This invention relates to a system and a method for controlling a towed seismic array comprising at least one deflector on one side of a towing vessel coupled through at least one wire, lead-in or similar, the deflector being positioned with a distance perpendicular to the towing vessels direction of movement, the towing vessel being provided with a navigation system for measuring the position of the vessel.
In seismic surveys at sea a number of seismic cables are usually being towed after a vessel. The survey of the geological formations at the sea bottom is performed by sending sound waves from one or more acoustic sources down into the sea bottom where they are reflected at the transitions between different types of formations. The reflected signals are received by sensors positioned in the seismic cables. The towed cable array is towed along a chosen path to perform the survey in a chosen area. The movements of the vessel and the array must be controlled precisely to secure a coverage of the wanted areas.
Accurate control of the vessels and arrays positions are especially important when the same reflection points are to be surveyed more than once to improve the measurements. This may be done by controlling the time between the emitted acoustic signals relative to the distance between the sensors and the vessels velocity so that the next signal is reflected from the reflection point up to a later sensor along the same cable.
To secure accurate measurements it is usual to monitor the position of the vessel using existing navigation systems to correct this. It has, however, become evident that it is difficult to obtain the required accuracy, among other reasons because of the size and momentum of the vessels. Different systems have also been used for compensating for the errors, e.g. signal treatment, over sampling or by controlling the deflectors pulling the array sideways out from the vessel, thus increasing the complexity of the system, e.g. as the control signals and power has to be transmitted from the vessel out to the deflectors.
U.S. Pat. No. 4,781,140 describes a system for compensating for the orientation of a vessel relative to the direction of movement. The seismic cables are coupled to rigid beams the orientation of which are changed using wires when the vessel for example has to be directed against the wind or current. It does, however, not provide any possibility for compensating for deviations in the vessels position from the predetermined path.
It is an object of the present invention to provide a simple system using existing navigation systems for compensating for drift and movements relative to a predetermined course, without making large demands to the manoeuvring systems of the vessel and to the systems treating the seismic data. It is also an object of the present invention to provide a system using simple and commercially available equipment position on the vessel.
It is a further object of the invention to provide a system and a method making it possible to maintain the seismic cables in a linear movement even if the towing vessel must perform manoeuvres departing from the predetermined direction within certain limits.
These objects are obtained using a system and a method as disclosed in the accompanying independent claims.